This application claims the priority of German patent document 100 10 070.8-41, filed Mar. 2, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a gas generator.
Fuel cells are more energy efficient than internal combustion engines, which is why fuel cells are increasingly used for electricity generation. This includes both stationary and mobile applications. Fuel cells are customarily operated using hydrogen. Since hydrogen is difficult to store, attempts are being made in the case of mobile applications, such as motor vehicles, to store the hydrogen in the form of liquid motor fuels. Examples of such motor fuels are pure hydrocarbons and alcohols. Mobile applications today predominantly utilize methanol, which is split by a gas generator into hydrogen and CO2. The hydrogen thus generated is then used to operate a fuel cell of a vehicle. However, disadvantages of using methanol are the still nonexistent methanol infrastructure and low storage density of methanol compared with petroleum-based motor fuels. Moreover, the high energy efficiency of a methanol fuel cell system is virtually cancelled out by the preceding methanol production. Hydrogen generation from conventional liquid power fuels such as gasoline, diesel or LPG is therefore an interesting alternative for a mobile fuel cell system. Such a fuel cell system comprises a fuel cell plus cooling media connection and air supply and also a gas generator.
U.S. Pat. No. 3,545,926 discloses a gas generator comprising a reforming reactor to produce a hydrogen-rich gas from a fuel, water and oxygen; a shift reactor to convert carbon monoxide with the aid of water into hydrogen; and a downstream gas-purifying unit. Both the reforming reactor and the shift reactor are supplied with water from an unspecified water reservoir vessel.
Such a gas generator has the disadvantage, especially in mobile use, for example when it is used to provide hydrogen for fuel cells in vehicles, that it needs to be provided with a water reservoir vessel which can freeze at ambient temperatures below freezing. Conventional antifreezes have a poisoning effect on reforming catalysts and so need to be removed prior to the metered addition, which represents an appreciable inconvenience and may even be impossible.
Against that background, it is an object of the present invention to provide a gas generator possessing reliable frost protection.
The present invention is based on the use of methanol as an antifreeze for the water reservoir vessel in a conventional gas generator. The water reservoir vessel has the function of providing water for the water gas shift reaction carried out in the shift reactor. Furthermore, in the case of gas generators which at least additionally perform steam reforming in the reforming reactor, the water reservoir vessel can also be used to supply the water for the steam reforming. The methanol can be used independently of the fuel used for the reforming reaction. That is, the reforming reaction itself may utilize methanol, but may also utilize any other suitable fuels, such as long-chain hydrocarbons and also higher alcohols, gasoline, diesel, LPG (liquefied petroleum gas) and NG (natural gas) or dimethyl ether.
The water reservoir vessel is thus stocked with a water-methanol mixture instead of pure water. Since the freezing point of such a water-methanol mixture depends on the mixing ratio, the desired degree of frost protection can be controlled by appropriately presetting a mixing ratio. The use of methanol as an antifreeze has the advantage that methanol can be converted into a hydrogen-rich gas in conventional shift reactors under customary operating conditions and over customary shift catalysts via a steam reforming reaction. This has the advantage not only of providing adequate frost protection and of protecting the catalysts against any damage, but also of improving the hydrogen yield in the gas generator.
The methanol in the water does not upset the reforming reactor either. When methanol is used as a primary fuel, the methanol in the water reservoir vessel will merely increase the methanol content in the reforming reactor. When other fuels are used, the methanol is converted into hydrogen at the same time as the actual fuel.